As a spark plug configured to ignite an air-fuel mixture, a spark plug that uses a non-equilibrium plasma is provided (Japanese Unexamined Patent Application Publication No. 2014-22341). In the spark plug disclosed in Japanese Unexamined Patent Application Publication No. 2014-22341, a metal shell holds a bottomed tubular insulator into which a center electrode is inserted. In this spark plug, when an AC voltage is applied between the metal shell and the center electrode or a pulse voltage is applied a plurality of times therebetween, electric charge generated based on a dielectric constant of the spark plug moves to the surface of the insulator and a gas around the insulator is ionized (a non-equilibrium plasma is generated around the insulator). Thus, the air-fuel mixture is ignited.
In the related art described above, however, a clearance is provided between the inner peripheral surface of the bottomed tubular insulator and the center electrode in order to secure efficiency of insertion of the center electrode into the insulator at the time of manufacturing the spark plug. The clearance, that is, an air layer formed between the insulator and the center electrode, is arranged between the center electrode and the metal shell in series with the insulator. Therefore, an apparent dielectric constant of the spark plug decreases. As a result, a problem arises in that the amount of electric charge generated on the surface of the insulator decreases relative to electric power input to the spark plug (a loss occurs).
The present invention has been made to address the problem described above and provides a spark plug in which the ignitability can be improved by suppressing a loss of energy.